1. Field of the Invention
The present invention relates to a masking film roll for use in painting, a method for producing it, and a tubular film from which it is produced. Precisely, the masking film roll for use in painting of the present invention is produced from a tubular film, of which the inner surface has been corona-discharged, and is widely utilized in painting constructions, pavements, airports, sports grounds, floors, walls, cars, ships, airplanes, vehicles, machines, office supplies, sports goods, amusement goods, leisure goods, displays, toys, woodwork, household articles, etc.
2. Prior Art
Various paintings are applied to many kinds of constructions, pavements, sports grounds, car parks, cars, vehicles and other various articles in order to make them have good appearance or to put thereon marks, ruled signs and illustrations. To attain such paintings, an area not to be painted with a coating material is previously masked with paper, synthetic paper, non-woven fabric, plastic film or the like by which it is easily separated from an area to be be painted. Accordingly, any direct application of a coating material to the surface of the area not to be painted therewith is prevented, and even when the coating material that has been applied to the area to be painted migrates to the area that shall not be painted, it is not in direct contact with the latter but is blocked by the paper, synthetic paper, non-woven fabric, plastic film or the like that has covered the area not to be painted. In this manner, the area that shall not be painted with a coating material is shielded and protected from being painted with the material. The above-mentioned paper and other films capable of exhibiting this function are referred to as masking films.
Various types of masking films have heretofore been employed, depending on the shape of the substance to be painted, the area to be painted, the properties of the coating material to be used, the temperature at which the coated material is baked, the temperature at which the coated material is dried, etc. The masking film to be provided by the present invention is intended to be directed to relatively large-sized paintings where the area to be painted is large and the area not to be painted (the area to be masked) is also large. In such paintings, the masking film need not always have the ability to adhere to or to temporarily adhere to the part to be not painted (the part to be masked) but needs to be such that, if a coating material has once been brought into contact with the surface of the masking film, it can firmly stick thereto with ease and therefore hardly runs thereon, and even after the coating material thus stuck to the masking film has been dried, it does not peel off from the surface of the film.
As the masking material of this type, paper, polyolefins and the like have been most popularly used in view of their costs and properties. However, paper is problematic in that its softness, water-proofness and solvent resistance are unsatisfactory, it is not easy to handle, it is not transparent and its mechanical strength is poor. For these reasons, the use of polyolefins that are free from such drawbacks in paper is increasing in place of paper.
One typical method of producing a masking film from a polyolefin film comprises, for example, inflating a high-density polyethylene having a density of from 0.94 to 0.97 g/ml, a melt index of from 0.05 to 0.8 g/10 min and a melt tension of 5 g or more into a film at an extrusion temperature of from 170 to 210.degree. C. and at a blow rate of from 1:2 to 1:5 to have a frost line of from 200 to 800 mm, taking up it at a taking-up speed of from 10 to 200 m/min to be a tubular film having a thickness of from 5 to 20 .mu.m, conveying it in the lengthwise direction in order while applying thereto a thin metal cutter at one site of the film to thereby make the film have a break in the lengthwise direction, then flattening the film with rollers, rotating one edge of the film, along which it has been cut, by 180 degrees around the lengthwise direction of the right-handed end or the left-handed end of the film as the rotation axis to thereby make the inner surface of the film face upside, then sticking an adhesive tape on the film along one edge thereof in the lengthwise direction in such a way that the adhesive tape is made to partly overlap with the edge of the film, and finally winding up the film around a paper tube into a roll.
However, since the surface of the masking film thus produced according to the method mentioned above is inactive, the components constituting a coating material do not easily adhere thereto. In particular, when the masking film is used while being attached to the perpendicular surface of a substance to be painted, the coating components that have scattered onto the masking film will run down to soil the surface of the floor to thereby worsen the working environment while those that have adhered to the surface of the film easily peel off therefrom, after having been dried thereon, to scatter around the floor to further worsen the working environment. For these reasons, the masking film is unfavorable.
On the other hand, one prior art technique of improving such a masking film has been disclosed in Japanese Patent Application Laid-Open No. Hei 3-42066 (JP-A 3-42066), by which a flat polyolefin film is subjected to corona-discharging to enhance the affinity of the treated surface of the film with a coating material. According to this technique, it is said that coating components can easily adhere to the surface of the film and therefore hardly peel off therefrom. Since the masking film to be produced from a polyolefin tubular film such as that mentioned above has been corona-discharged, it may exhibit the above-mentioned effects in some degree and has already been commercialized. Referring to Example 1 in the specification of the publication, the masking film is produced by "flattening a tubular film with pinch rollers and subjecting the flattened film to corona-discharging in air just before winding up it around a winding-up roll, whereupon the conditions for the corona-discharging to be conducted by the use of a corona-discharging device of HFSS-101 Model ex Kasuga Electric Co. are such that the electrode-film distance is from 2 to 3 mm". The corona-discharging employed is an ordinary one that has heretofore been applied to polyolefin tubular films, by which it is obvious that only the outer surface of the tubular film is corona-discharged but the inner surface thereof is not influenced at all.
The present inventor has actually produced a masking film according to the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 3-42066 and have found that the method as well as the product produced according to the method have various problems. First, according to the method, a tubular film is flattened, then the outer surface of the thus-flattened film is subjected to corona-discharging, the film is cut open to be an open flat film, and an adhesive tape having a width of 15 mm is lengthwise laminated onto the corona-discharged surface of the film in such a way that a part (having a width of about 4 mm) of the adhesive layer of the tape may lengthwise overlap with the corona-discharged surface of the film by passing the film along with the tape between rollers. The film thus produced is not folded in the widthwise direction before packaging orrolling it for commercial sale. If the film is narrow, it may be directly rolled into a roll film without involving any problem in bulkiness. However, such a narrow masking film is in little demand but wide masking films having a width of from 20 to 250 cm and capable of covering large area are in great demand.
If a wide film is produced according to the method disclosed in Japanese Patent Application Laid-Open No. Hei 3-42006, it shall be rolled as it is wide. Therefore, the width of the resulting roll film is large so that the space for storing the roll films shall be large. In addition, the operation for sticking the adhesive layer part of the masking film on the surface to be masked with the film is difficult, and, after a part of the film has been stuck on the surface to be masked therewith, the operation for tearing off the excessive film by hand is also difficult. In order to solve these problems, the film shall not be handled with open width but shall be folded plural times in parallel in the lengthwise direction before use. After having been thus folded, the film was made to be free from the drawback in the large width thereof. However, the adhesive surface of the adhesive tape as adhered to the masking film is on the same plane as the corona-discharged surface of the film that is positioned along the edge of the tape in the widthwise direction as seen from the adhered part of the tape, though there is a distance of several centimeters therebetween. Therefore, while the masking film roll is unrolled and is applied to a substance to be masked therewith, the edge of the corona-discharged film being unrolled is trailed by the adhesive tape along its adhesive surface. For these reasons, the operation of unrolling and applying the masking film to the substance to be masked therewith is extremely difficult.
In order to evade this trouble, the tubular film is cut open after its outer surface has been corona-discharged, then the film is turned upside down in order that the corona-discharged surface of the film is made to face inside while the non-corona-discharged surface thereof is to face outside, thereafter the film is folded in parallel in the lengthwise direction in the same manner as above, and an adhesive tape is stuck on the corona-discharged surface of the film along its edge. If so, the adhesive surface of the adhesive tape is prevented from being positioned adjacent to and on the same plane as the non-corona-discharged surface of the masking film along the opposite edge of the film, but the non-corona-discharged surface of the film can be positioned in the opposite side to the adhesive surface of the tape in such a way that the non-corona-discharged surface of the film faces outside. Therefore, when the masking film having an adhesive tape in this manner is applied to the surface of a substance to be masked therewith, the edge of the film is prevented from being trailed by the adhesive tape.
As has been described hereinabove, the up-to-date prior art technique for producing a compact masking film roll from a polyolefin is such that a tubular film is first made from a polyolefin, its surface is corona-discharged, the tubular film is then cut open and then turned upside down, then the film is folded several times in the lengthwise direction, and an adhesive tape is stuck on the film in such a way that the adhesive surface of the tape is made to face the corona-discharged surface of the film. The handleability of the masking film to be produced according to the prior art technique mentioned above is good. However, the process itself for producing the masking film is extremely complicated and therefore the masking film rolls to be produced according to the process could not be put to practical use in commercial base.
Given the situation as above, the subject matter of the present invention is to provide a method for economically and inexpensively producing a masking film roll that does not need any large space for storing it and that can easily be applied to a substance to be masked therewith, as well as the masking roll film itself to be produced according to the method, and also to provide a tubular film from which the masking film is produced.